The present invention relates to the field of custom part making, and particularly to the manufacture of custom parts which involve a CNC machining operation based upon a CAD file provided by the customer. Such parts include parts which are made from CNC machined molds, such as for use with injection molding presses, from blocks of metal, and parts which are directly CNC machined from blocks of workpiece material. More specifically, the present invention relates to software supported methods, systems and tools used in the design and fabrication of such custom plastic parts formed utilizing a CNC machining operation, and in presenting information to customers for the customer to have selective input into various aspects of such design and fabrication which affect price of a customized part profile.
Injection molding, among other types of molding techniques, is commonly utilized to produce plastic parts from molds. Once the injection mold is created and the injection mold press is properly set up, injection molding can quickly create parts of complex geometries in quick succession to reach high-volume runs. Companies and individuals engaged in fabricating molds are commonly referred to as “moldmakers.” In many cases (referred to as “straight pull” injection molding), the mold consists of two metal blocks, one top and one bottom. Most commonly, the metal blocks are high quality machine steel, so the mold will have an acceptably long life. Opposed surfaces of each mold block are machined to jointly produce the required cavity in the shape of the desired part, as well as “shut-off” surfaces sealing the cavity when the mold blocks are pressed together. The line on which shut-off surfaces intersect with the surface of the cavity is called the parting line. The corresponding line on the surface of the part formed by the parting line is called the witness mark. After the mold assembly is set up in an injection molding press, parts are made by filling the cavity with molten plastic. The mold blocks are separated from each other after solidification of the molten plastic. The plastic part, normally sticking after separation to the bottom block, is then ejected by means of ejectors.
The moldmaking art has a long history of fairly gradual innovation and advancement. Molds are designed pursuant to a specification of the part geometry provided by a customer; in many cases, functional aspects of the plastic part also need to be taken into account. Historically, moldmaking involves at least one face-to-face meeting between the moldmaker and the customer, in which the customer submits detailed part geometry, usually with the aid of drawings, to the moldmaker and outlines the function of the part. Armed with knowledge of injection molding technology, the moldmaker designs the mold corresponding to the drawings of the part. In particular, the moldmaker orients the part to enable a straight pull mold separation, splits its surface into two areas separated by a suitable parting line, and replicates these areas in the top and bottom blocks. The moldmaker determines the location and shape of the shut-off surfaces and enlarges the dimensions of the cavity relative to the desired part as necessary to account for shrinkage of the plastic material. The moldmaker determines the size and position of one or more gates and runners to provide an adequate flow path for the molten plastic shot into the cavity. Sizes and locations of openings for ejection pins are also selected by the moldmaker. The machining operations to be performed to fabricate the designed mold are determined by the moldmaker. The moldmaker then runs various cutting tools, such as endmills, drills and reams, to machine the basic cavity, shut-off surfaces, runners, gates and ejector pin openings in blocks of metal. To produce certain hard-to-mill features in the mold, the moldmaker may also design and machine electrodes, and then perform electro-discharge machining (“EDM”) of the mold blocks. The moldmaker then outfits the mold blocks with ejection pins and prepares the mold assembly for use in the injection molding press. Throughout all of this design and fabrication, the moldmaker makes numerous design choices pertaining to the geometric details of the cavities to be machined as well as to the tools to be used for machining.
Beyond using machining to form an injection mold, machining has long been used to directly shape metal, wood, plastic and similar solid materials into parts. Machining involves a subtractive process, wherein material is reamed, drilled, sawed, lathed, cut in chips or similarly removed from a larger solid block which is held or fixtured in to the tool. CNC machining has accelerated the machining process and become commonplace in many part making and machine shops. CNC machining requires the writing of code instructing the CNC machine which tools and tool paths are needed for the material removal steps. Just as when CNC machining is used to machine a mold block, the process for generating CNC tool paths to directly machine a part can be simple or difficult depending upon the complexity of the tool paths. For simple profiles, typically having a rectangular or box-like shape which can be readily held with vices on the CNC machine, CNC machining may be a viable option, either in low-, mid- or high-volume runs. As part shape profiles and geometries are designed to be more complicated, CNC machining often requires the creation of custom fixtures for holding the part during machining. It is not unusual for the design and fabrication of the custom fixturing to involve more time and expense than the design and fabrication of a single part. With the added time and complexity associated with custom fixturing, CNC machining is rarely used for low-volumes of parts having more complex shapes which need to be fabricated in a quick turn-around time. For parts in mid- or high-volume runs, the design and fabrication of custom fixtures may be warranted, making machining again a viable option depending upon part shape. Even with custom fixturing, if the machining time for the part takes too long, often other methods of part manufacture will be more cost effective than “total profile” machining, i.e., machining a substantial majority of the surface area of the part in the CNC machine.
Regardless of whether a mold is formed or a part is directly machined, all these steps involve a high degree of skill and experience on the part of the moldmaker and/or machinist. Experienced moldmakers and/or machinists, after having considered the design submitted by the customer, may sometimes suggest changes to the part geometry so that the part is more manufacturable and less costly. Highly experienced, gifted moldmakers and machinists can charge a premium for their services, both in return for the acuity of their experience and perception in knowing what will and will not work in the mold, and in return for their skill, speed and craftsmanship in machining the mold or directly machining the part.
Because of the large number of technical decisions involved and considerable time spent by highly skilled moldmakers in analyzing in detail the part geometry by visual inspection, obtaining a desired injection mold has generally been quite expensive and involved a significant time delay. A single mold may cost tens or hundreds of thousands of dollars, and delivery times of eight to twelve weeks or more are common. While often not quite as time consuming or costly as machining a mold, obtaining even a single part from a highly skilled machinist can also involve great expense and significant time delay.
As in many other areas of industry, various computer advances have been applied to the moldmaking and machining arts. Today, most of customer's drawings are not prepared by hand, but rather through commercially available programs referred to as CAD (Computer-Aided Design) software. To produce drawings of the molds based on the drawings of custom parts, moldmakers also use CAD software, including packages developed specifically for this task. Also, in most moldmaking companies and in many machining companies, machining operations are not manually controlled. Instead, CNC (Computer Numerical Control) machines such as vertical mills are used to manufacture parts, molds and/or EDM electrodes in accordance with a set of CNC instructions. To compute detailed toolpaths for the tools assigned by the moldmaker or machinist and to produce long sequences of such instructions for CNC mills, computers running CAM (Computer-Aided Manufacturing) software (again, including some packages developed specifically for the moldmaking industry) are used by many moldmakers and machinists. CAD/CAM software packages are built around geometry kernels—computationally intensive software implementing numerical algorithms to solve a broad set of mathematical problems associated with analysis of geometrical and topological properties of three-dimensional (3D) objects, such as faces and edges of 3D bodies, as well as with generation of new, derivative 3D objects. At present, a number of mature and powerful geometry kernels are commercially available.
While existing CAD/CAM software packages allow designers and CNC machinists to work with geometrically complex parts, they are still far from completely automating the designer's work. Rather, these packages provide an assortment of software-supported operations that automate many partial tasks but still require that numerous decisions be made by the user to create the design and generate machining instructions. CAD/CAM packages usually facilitate such decisions by means of interactive visualization of the design geometry and machining tools. This makes software applicable to a wide variety of tasks involving mechanical design and machining operations. The downside of such versatility, when applied to moldmaking and machining, is that it results in long and labor intensive working sessions to produce mold designs and CNC machining instructions for many custom parts, including parts lending themselves to straight pull molding.
Visualization allows the moldmaker to evaluate whether the mold and injection molded parts can be made sufficiently close to the design using available tools. The fidelity with which plastic parts can be manufactured is limited by the finite precision of mills and cutting tools used to machine the part or the mold. The fidelity with which plastic parts can be molded may be further limited by the shrinkage of plastic materials (slightly changing the shape and dimensions of the injection molded parts as they cool down and undergo stress relaxation in a way that is largely but not entirely predictable). These rather generic factors establish the level of dimensional tolerances for directly machined or injection-molded parts, the level that is generally known and in most cases acceptable to the customers.
Oftentimes, however, additional factors come into play that can result in more significant deviations of injection molded plastic parts or directly machined parts from the submitted design geometry. These factors are usually associated with certain features that are hard to machine using vertical mills. For example, very thin ribs in a molded part can be made by cutting deep and narrow grooves in the mold, but may require an endmill with an impractically large length to diameter ratio. Machining deep and narrow grooves directly in the part may similarly require an endmill with an impractically large length to diameter ratio. Machining of angles between adjacent faces joined by small radius fillets (and, especially, of angles left without a fillet) may result in similar difficulties. Exact rendering of such features may substantially increase the cost of the part or the mold, and even make its fabrication impractical with the technology available to the moldmaker/machinist.
Obviously, such manufacturability issues need to be identified, communicated to the customer, and, if necessary, rectified before proceeding with mold or part fabrication. Their resolution normally requires tight interaction between the moldmaker/machinist and the customer, as both parties are in possession of complementary pieces of information needed to resolve the issues. The moldmaker/machinist has first hand knowledge of the mold/part fabrication technology available to him, while the customer, usually represented in this process by the part designer, has first hand understanding of part functionality and cosmetic requirements. Based on this understanding, the customer can either agree to the anticipated deviations of part geometry from the submitted specification, or, if the deviations are unacceptable, the customer can modify the part design to resolve manufacturability issues without compromising functional and cosmetic aspects of the design.
As custom parts often have many unnamed (and hard to name) features, pure verbal communication not supported by visualization of the part can be awkward and misleading. Therefore, communicating such information requires a face-to-face meeting with the customer, in which the moldmaker/machinist and the customer view the drawing or image of the part and discuss the issues in detail. Such meetings take a considerable amount of time, both for moldmakers/machinists and their customers, and increase business costs.
Resolution of manufacturability issues is closely connected with price quotations requested by customers. When a customer requests a price quotation for a directly machined part or a molding project, the machinist/moldmaker informally applies a wealth of experience and knowledge to predict costs and various difficulties in fabricating the part or the mold. The potential manufacturability issues should be substantially resolved before a binding quotation can be given to a customer. For this reason, it can often take one or two weeks for a customer just to obtain a price quotation. Quoting is performed at a stage when securing the order for the job is uncertain, and the cost of quoting must be recovered by the moldmaker/machinist from the subset of quotes that are actually accepted.
In the event that the customer contracts with the moldmaker/machinist for the job, the quotation becomes a constituent part of the contract for manufacturing the part(s) and/or the mold. For obvious reasons the informal quoting method is prone to human errors. If the request for quotation results in the job order, such errors will most likely become apparent during part or mold design and machining, or even after the mold is finished and used for manufacturing the first plastic parts. The price of such mistakes in terms of the lost time and effort, as well as in terms of strained customer relations, may be rather high. Thus, for a moldmaking or machining business to be successful and profitable, good communication between the customer and the moldmaker/machinist in resolving manufacturability issues and accurate quoting are extremely important.